1. Field of the Invention
The present invention relates to a technique for encrypting data.
2. Description of the Related Art
Conventionally, a technique is known in which, in order to store sensitive information in a storage device in the form of electronic data, the important data is encrypted by using a predetermined encryption algorithm to store the encrypted important data in the storage device. Another technique is also known in which, in a case of, for example, discarding the storage device, the important data is overwritten and stored with a predetermined data (e.g., a data string including only zeros) in order for the important data stored in the storage device not to be read out any more.
For example, all the pieces of data to be input are considered as the important data and therefore, a user considers to store the important data in the storage device after surely encrypting it. In the case, it is desirable to constitute a system such that the important data input in a device is always processed through an encryption processing unit before it is stored.
However, in the above described system, if the user attempts to overwrite the important data with the predetermined data (hereinafter, referred to as the “fixed data”) and store thereof, since the fixed data is also stored in the storage device after it is encrypted, a purpose of “overwriting and storing data by using the fixed data” is not achieved.
An example of simpler method includes a system in which the fixed data bypasses the encryption processing unit only in a case of the fixed data. However, in this case, data that can be acquired by the encryption by the encryption processing unit is identical to the fixed data may be generated. Therefore, the data cannot be discriminated whether the data is acquired by overwriting it by using the fixed data or the data is acquired by encrypting the data. Accordingly, data acquired by encryption by using the encryption processing unit and identical to the fixed data cannot be correctly decrypted.